ARTICLE IN PRESS

Journal of Luminescence 129 (2009) 874–878

Contents lists available at ScienceDirect

Journal of Luminescence

0022-23

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Synthesis, effect of capping agents, structural, optical and photoluminescenceproperties of ZnO nanoparticles

A.K. Singh �, V. Viswanath, V.C. Janu

Defence Institute of Advanced Technology, Girinagar, Pune 411025, India

a r t i c l e i n f o

Article history:

Received 28 June 2008

Received in revised form

13 February 2009

Accepted 21 March 2009Available online 5 April 2009

PACS:

71.35.Cc

78.55.Et

78.55m

Keywords:

ZnO

Capping agent

UV–vis

Photoluminescence

13/$ - see front matter & 2009 Elsevier B.V. A

016/j.jlumin.2009.03.027

esponding author. Tel.: +91 2024304173; fax:

ail address: draksingh@hotmail.com (A.K. Sin

a b s t r a c t

Zinc oxide nanoparticles were synthesized using chemical method in alcohol base. During synthesis

three capping agents, i.e. triethanolamine (TEA), oleic acid and thioglycerol, were used and the effect of

concentrations was analyzed for their effectiveness in limiting the particle growth. Thermal stability of

ZnO nanoparticles prepared using TEA, oleic acid and thioglycerol capping agents, was studied using

thermogravimetric analyzer (TGA). ZnO nanoparticles capped with TEA showed maximum weight loss.

X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for structural and

morphological characterization of ZnO nanoparticles. Particle size was evaluated using effective mass

approximation method from UV–vis spectroscopy and Scherrer’s formula from XRD patterns. XRD

analysis revealed single crystal ZnO nanoparticles of size 12–20 nm in case of TEA capping. TEA, oleic

acid and thioglycerol capped synthesized ZnO nanoparticles were investigated at room temperature

photoluminescence for three excitation wavelengths i.e. 304, 322 and 325 nm, showing strong peaks at

about 471 nm when excited at 322 and 325 nm whereas strong peak was observed at 411 for 304 nm

excitation.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Zinc oxide (ZnO) is a wide band gap semiconductor, havinghigh exciton binding energy of 60 meV and has stable wurtzitestructure with lattice spacing a ¼ 0.325 nm and c ¼ 0.521 nm. Ithas attracted intensive research efforts for its unique propertiesand versatile applications in antireflection coatings, transparentelectrodes in solar cells, ultraviolet (UV) light emitters, diodelasers, varistors, piezoelectric devices, spin-electronics, surfaceacoustic wave propagator [1–3], antibacterial agent [4], photonicmaterial [5] and for gas sensing [6]. Invisible thin film transistors(TFTs) using ZnO as an active channel, have shown much higherfield effect mobility than amorphous silicon TFTs. These transis-tors can be widely used for display applications. ZnO has beenproposed to be a more promising UV emitting phosphor than GaNbecause of its larger exciton binding energy leading to a reducedUV lasing threshold and yielding higher UV emitting efficiency atroom temperature [7]. Surface acoustic wave filters using ZnOfilms have already been used for video and radio frequencycircuits. Bulk and thin films of ZnO nanoparticles have demon-strated high sensitivity to toxic gases [8]. Several authors havereported high photoluminescence efficiencies in ZnO nanostruc-

ll rights reserved.

+91 2024389411.

gh).

tures [9,10]. Also, ZnO is an environment friendly material, whichis desirable especially for bioapplications. Due to the increasingdemands globally for green materials and processes, new andefficient synthesis processes are desired. A large number oftechniques like spray pyrolysis, thermal decomposition, chemicalvapour deposition, laser ablation, etc. have been used forsynthesis of nanomaterials and structures. Chemical synthesis isone of the important techniques which can be performed using arange of precursors and synthesis conditions like temperature,time, concentration, pH of reactants, etc. Optimization of theseparameters leads to nanoparticles of different size, shapes andshowing different optical properties.

In this study, ZnO nanoparticles are synthesized by wetchemical method, which is a simple method, under ambientatmosphere at room temperature, and characterized by UV–vis,X-ray diffraction (XRD), and scanning electron microscopy (SEM).Concentration of TEA is varied and also the capping agent ischanged to oleic acid and thioglycerol and its effect on the sizeand the properties of the particles is analyzed.

2. Synthesis methodology

Zincacetatedihydrate, Zn(Ac)2 �2H2O dissolved in dimethyl-sulpoxide (DMSO) and potassiumhydroxide (KOH) dissolved inethanol is used to synthesize ZnO and then triethanolamine (TEA)

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A.K. Singh et al. / Journal of Luminescence 129 (2009) 874–878 875

is added as capping agent. Zincacetatedihydrate of 0.2 M,Zn(Ac)2 �2H2O, is prepared in 20 ml dimethylsulpoxide and stirredtill it is completely dissolved and forms a clear solution.Potassium hydroxide solution of 1.2 M in 10 ml ethanol is addedto the solution of zinc acetate drop wise till the solution becomesmilky white under slow stirring condition until it is uniformlywhite. Intended capping agents, i.e. TEA, oleic acid, thioglycerol, inrequired concentration, is added and stirring is continued forproper mixing of the capping agent in the milky white solution.The precipitate is separated by centrifugation and then washed atleast three times and then naturally dried to get fine whitepowder. All the chemicals used were purchased from the leadingsuppliers without further purification.

0.4

0.5

0.6

0.7

0.8

0.9

1TEA 0.24 ml (double)

TEA 6.0 ml (50 times)

TEA 0.12 ml

TEA 9.0 ml (75 times)

bsor

ptio

n (a

.u.)

3. Measurements and analysis

UV–vis absorption study has been carried out using Nanodrop1000 spectrophotometer by dispersing nanoparticles in methanol.Photoluminescence (PL) measurements have been done at roomtemperature using Perkin Elmer (LS-55) Luminescence Spectro-photometer. Thermogravimetric analysis (TGA) has been doneusing Du Pont analyzer, model 951. Analysis of crystal structure,crystal size and morphology has been carried out by X-raydiffraction and scanning electron microscopy. Stoichiometricanalysis has been done using EDAX. The energy correspondingto the exciton absorption peak has been converted in terms ofparticle size using the effective mass approximation [11]

E ¼ Eg þ h2p2 1

meþ

1

mh

� ��ð1:8e2Þ

4p�0�00Rþ Smaller term (1)

where E is band gap of synthesized particle, Eg is bulk band gap ofZnO (3.3 eV), R is radius of the particle, me is effective mass ofelectron (0.28 mo), mh is effective mass of the hole (0.49 mo), e0 isdielectric constant of material (9.1), e00 is permittivity of freespace, h is Planck’s constant. From Scherrer’s formula [12] usingFWHM of XRD patterns, size of particles is

D ¼0:9lb cosy

(2)

where l is wavelength of X-ray source, b is full-width at half-maximum in radians, y is Bragg’s diffraction angle.

0

0.1

0.2

0.3

200

A

Wavelength (nm)300 400 500 600 700 800

Fig. 1. UV–vis absorption spectra of ZnO powder capped with 0.12, 0.24, 6.0 and

9.0 ml concentrations of TEA.

3. Results and discussion

Table 1 summarizes the synthesis results of ZnO, effect ofcapping agents on excitonic peaks and particle size estimatedfrom mass approximation Eq. (1). Fig. 1 shows the UV absorptionspectra of ZnO capped with different concentrations of TEA.Absorption peaks corresponding to 0.12, 0.24, 6.0 and 9.0 mlconcentrations of TEA are obtained at 360, 357, 338 and 337 nm,

Table 1Summary of synthesis of results and effect of capping agents on UV absorption peak a

S. No. Capping agent Qu

ag

1 Triethanolamine (TEA) 0.1

2 Triethanolamine (TEA) 0.2

3 Triethanolamine (TEA) 6 (

4 Triethanolamine (TEA) 9 (

5 Oleicacid 0.2

6 Thioglycerol 0.1

7 Annealing at 423 K for 3 h for sample ZnO with TEA (S. No. 1

8 Solution temperature raised to 70 1C for ZnO with TEA as cap

respectively. From Eq. (1) size of the particle is found to bebetween 30 and 4 nm. As seen from Fig. 1 and Table 1, theabsorption spectra for all the samples are blue shifted to, from375 nm absorption wavelength (expected for bulk ZnO havingdirect band gap of 3.3 eV) and showing sharp excitonic peaks.

3.1. Effect of TEA concentration

To analyze the effect of concentration of capping agent,concentration of TEA is increased i.e. 2, 50 and 75 times of theinitial concentration (0.12 ml). Observation of Fig. 1 shows thatthere is gradual ‘‘blue shift’’ in the UV absorption spectra,correspondingly the size of ZnO has decreased to 15–20, 3–4and 3–4 nm, respectively, as obtained from the mass approxima-tion. Observation of Table 1 also shows that the effect of increasein concentration of TEA is only up to certain level i.e. 50 times,after which increase in TEA concentration becomes ineffective.

3.2. Effect of capping agents on the size of particle

To analyze the effect of various capping agents, TEA has beenreplaced first with oleic acid and then with thioglycerol. Theconcentration used is 0.24 ml of oleic acid and 0.12 ml ofthioglycerol, respectively, and the UV absorption spectra observedis shown in Fig. 2 for all the samples. Observation of figure showsthat thioglycerol is more effective capping agent than TEA. It isseen that to achieve 3–4 nm size of ZnO, 9.0 ml of TEA is requiredor 0.24 ml of oleic acid against 0.12 ml of thioglycerol. This can beattributed to the greater steric effect on nitrogen of TEA, which

nd particle size.

antity of capping

ent (ml)

UV absorption

peak (nm)

Size of particles obtained

from Eq. (1) (nm)

2 360 20–30

4 (2 times) 357 15–25

50 times) 338 3–4

75 times) 337 3–4

4 344 4–5

2 335 �3

) 364 40–45

ping agent (S. No. 1) 350 15–20

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

200

Capping agent: Oleicacid 0.24ml

Capping agent : Thioglycerol 0.12ml

Capping agent:TEA 0.12 ml

Wavelength (nm)

Abs

orpt

ion

(a.u

.)

300 400 500 600 700 800

Fig. 2. Comparison of UV–vis absorption spectra with different capping agents i.e.

TEA, oleic acid, thioglycerol.

86

88

90

92

94

96

98

100

102

104

106

0

ZnO nanoparticles capped with TEA

ZnO nanoparticles capped with Oleicacid

ZnO nanoparticles capped with Thioglycerol

ZnO commercial

Wei

ght L

oss

(%)

Temperature (°C)200 400 600 800 1000

Fig. 3. Thermograph of commercial ZnO powder and ZnO nanoparticles capped

with TEA, oleic acid, thioglycerol.

0

20

40

60

80

100

0

TEAThioglycerolOleic acid

Wei

ght L

oss

(%)

Temperature (°C) 100 200 300 400 500 600

Fig. 4. Thermograph of capping agents.

200

100

200

300

400

500

600

700

(002) (112)

(103)(110)

(102)

(101)

Inte

nsity

(a.u

.)

(2θ)°

(100)

30 40 50 60 70 80

Fig. 5. X-ray diffractogram of zinc oxide nanoparticles capped with 0.12 ml of TEA.

A.K. Singh et al. / Journal of Luminescence 129 (2009) 874–878876

makes it less effective capping agent as compared to sulphur ofthioglycerol which is lesser hindered. The size of sulphur inthioglycerol is also larger than nitrogen of TEA which makes itmore effective capping agent as compared to TEA. TGA of ZnOnanoparticles prepared using various capping agents is shown inFig. 3 along with the thermograph of pure ZnO powder whereasFig. 4 shows the thermograph of capping agents. TGA ofnanoparticles and of capping agents not only providesinformation about the stability of nanoparticles but can also beused to evaluate yield of ZnO in final product [13]. ZnOnanoparticles capped with TEA showed maximum weight loss.

3.3. Crystal structure and morphology

Before discussing photoluminescence of ZnO nanoparticles it isuseful to consider their crystal structure and morphology. Fig. 5shows the X-ray diffraction pattern of 0.12 ml TEA capped ZnOnanoparticle. These peaks at scattering angles (2y) of 31.3, 33.1,35, 46.3, 55.3, 61.6 and 66.7 correspond to the reflection from:100, 002, 101, 102, 110, 103 and 112 crystal planes, respectively.

XRD of the ZnO nanoparticles indicates that they possess ahexagonal wurtzite type of crystallographic stricter. The XRDpattern shows broadening of the peaks indicating ultra-finenature of the crystallites. The peaks assigned to diffractionsfrom various planes correspond to hexagonal structure of ZnO.The crystallite size estimated for the same sample from Scherrer’sformula using FWHM [12] from XRD patterns is of the order of12–20 nm from different peaks, giving average size of 15.4 nmfrom all the peaks. Fig. 6 shows the energy dispersive analysis ofX-rays (EDAX) used to find the composition of zinc oxide sample.It shows zinc and oxygen to be almost in stoichiometric ratio.Fig. 7 shows SEM images of ZnO capped with 0.12 ml of TEA fortwo magnifications. Observation of figure shows that ZnOparticles are spherical in nature and size of the particles isin the range 40–50 nm (size resolved from SEM is about 50 nm,Fig. 5b).

3.4. Photoluminescence

Generally, ZnO shows four PL emissions [14]: (a) near bandedge emission at around 390 nm (UV emission), attributed to free-exciton recombination (b) blue emission at around 460 nm isbecause of intrinsic defects such as oxygen and zinc interstitials(c) green emission at around 540 nm is known to be a deep levelemission which is caused by impurities, a structural defects in thecrystal such as oxygen vacancies, zinc interstitials, etc. and (d) redemission at around 630 nm due to oxygen and zinc anti-sites.Fig. 8 shows the room temperature photoluminescence spectraof ZnO nanoparticles excited at three different wavelengths

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Fig. 6. EDAX of ZnO nanoparticles capped with 0.12 ml of TEA.

Fig. 7. SEM of the ZnO nanoparticles obtained with 0.12 ml of TEA capped agent.

0

50

100

150

200

250

300

350

350

Excitation at 325 nmExcitation at 322 nmExcitation at 304 nm

Wavelength (nm)

Inte

nsity

(a.u

.)

400 450 500 550 600 650 700 750 800

Fig. 8. PL emission spectra of ZnO nanoparticles capped with TEA at 304, 322 and

325 nm excitation wavelengths.

470.5

517.5

471.5

409.5

392.5

0

50

100

150

200

250

300

350

300

ZnO capped with TEA

ZnO capped with Thioglycerol

Wavelength (nm)

Inte

nsity

(a.u

.)

400 500 600 700 800

Fig. 9. Room temperature PL emission spectra of ZnO nanoparticles capped with

TEA and thioglycerol.

A.K. Singh et al. / Journal of Luminescence 129 (2009) 874–878 877

i.e. 304, 322 and 325 nm. At later two excitation wavelengths,photoluminescence spectra are found to be of similar natureshowing strong peak at about 471 nm. ZnO nanoparticles excitedat 325 nm exhibited four prominent emission PL bands at around390, 411, 471 and 517 nm. The PL results as shown in Fig. 8 aresimilar to those reported previously [14,15]. The PL peak at 411 isattributed to zinc vacancies [15] which is intense and sharp whenexcited with 304 nm. This peak intensity is increased at theexpense of band edge emission. This showed that the probability

of trapping and giving the PL emission by zinc vacancies at theexcitation of 304 nm is more as compared to 325 nm excitation.The peak around 470 nm i.e. blue emission is attributed tointrinsic defects such as oxygen and zinc interstitials [14]. Thisblue emission is common for all excitation wavelengths, but for322 and 325 nm excitation blue emission is prominent. The green

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A.K. Singh et al. / Journal of Luminescence 129 (2009) 874–878878

band emission corresponds to the singly ionized oxygen vacancyin ZnO in the bulk of nanoparticles and excess oxygen on thesurface, which might be in the form of OH� ions and results fromthe recombination of photo-generated hole with single ionizedcharge state of this defect [13,16]. The weak green emission alsoimplies that there are few surface defects in ZnO nanoparticles.

Fig. 9 shows the room temperature photoluminescence spectraof ZnO nanoparticles excited at 322 nm wavelength for twodifferent surfactants in same quantity (0.12 ml), i.e. TEA andthioglycerol, indicating the effect of particle size on PL intensity. Itshows that PL intensity is higher when thioglycerol is used assurfactant due to the formation of smaller ZnO nanoparticles ascompared to TEA. However, no blue shift has been observed inphotoluminescence spectra due to size effect. Similar results havebeen reported by Ghosh et al. [14], Tan et al. [17]. This is becausethe PL emission generally comes from the ZnO nanocrystals, andthe blue shift in the optical absorption spectra is due to theamorphous phase in the material [17].

4. Conclusion

ZnO nanoparticles have been successfully synthesized usingzinc acetate, DMSO and KOH in ethanol at room temperatureusing three capping agents, i.e. TEA, oleic acid and thioglycerol. Ithas been found that thioglycerol is more effective capping agentas compared to oleic acid and TEA, giving nanoparticles of size3 nm. ZnO nanoparticles capped with thioglycerol show highintensity luminescence. Blue emission at 471 nm is common forall excitation wavelengths, but for 322 and 325 nm excitation blueemission is prominent whereas for 304 nm excitation, emission at411 is prominent.

Acknowledgements

Authors are thankful to Vice-Chancellor, DIAT, Pune for grantingpermission to publish this work. Authors would like to thank Prof.SK Kulkarni, Department of Physics, University of Pune, forproviding Photoluminescence facility and technical discussions;and Director, DMSRDE, Kanpur, for SEM and EDAX of samples.

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